A breeding population would have to consist of more than 10 individuals, but there has been some speculation that these animals might actually migrate from open ocean to the lake to breed or something like that, that seems improbable as well.
then the question changes to : what is the likelihood that a group of substantial animals migrates up and down the river through Inverness without ever being seen ?
I would have to say that in modern times probably zero. Before the river was fitted with locks and other impediments it might have been possible.
Bull sharks migrate upstream in central america to a large lake, at one time the sharks in the lake were considered to be a separate species because it was thought the extreme rapids the sharks would have to swim through were impassible to such large fish but now we know they do it regularly.
I'm going to stick with the giant eel idea for these lake monsters, makes much more sense than extant plesiosaurs. http://en.wikipedia.org/wiki/List_of_re ... e_monstershttp://en.wikipedia.org/wiki/Champ_(cryptozoology
Lake Monsters are interesting in that it is possible they exist but are probably not "dinosaurs" or some other extinct animal surviving to modern times but there are other possibilities.
During colonial times a small creek in Ohio was devastated by a forest fire in the surrounding woods and in the aftermath several huge salamanders in the 5 to 10 foot range were found dead, no other specimens were ever found. At that time the area was wild and covered by old growth forests, subsequent deforestation destroyed the stream and it's inhabitants even the well known ones.
There have been reports of crocodilian type creatures in northern lakes that are far too cold to allow such creature to exist but very large unknown salamanders could and may be the basis for those sightings.
Eels are also a possibility, normal american and european eels do sometimes lack the urge to go back to their spawning grounds and can get very large. While no monster sized eels have ever been found it is not outside the range of reason to think such an eel that has no drive to reproduce, various reasons for this, could live many years and they never stop growing.
Some sightings, especially in the ocean, almost have to be some sort of cryptic creature. Many reports sound very much like plesiosaurs but there were ancient whales that fit the description as well and while unlikely are more likely than plesiosaurs. http://en.wikipedia.org/wiki/Basilosaurus
5. Tanystropheus: This hypothesis was proposed by Champ researcher Dennis Hall, who claims to have seen Champ 20 times. According to Hall, in 1976, his father caught a strange-looking reptile, on the shore of Lake Champlain. He then took it to scientists, who concluded that it was unlike any known species of living reptile. Unfortunately, however, this specimen was later lost. Hall then saw a picture of a Tanystropheus, and concluded that it was the most likely candidate, for Champ. However, there are numerous problems, with this hypothesis, as well. This is because Tanystropheus was a very specialized species of aquatic reptile, from the Triassic Period. This, therefore, makes it very unlikely, that it could have survived all the way to the present-day, and still inhabits Lake Champlain.
The above is from the link on Champ but the following link describes the extinct reptile it is suggested to be.http://en.wikipedia.org/wiki/Tanystropheus
More on lake beasts.http://www.angelfire.com/bc2/cryptodomi ... easts.html
Other possibilities but not all are freshwater.
Then you have to take in account the very real and very large freshwater fishes...